1. Field of the Invention
The present invention relates to a process and an apparatus for growing a silicon epitaxial layer on a silicon to substrate wafer.
2. Description of the Related Art
FIG. 7 is a schematic diagram illustrative of a longitudinal sectional side view of a prior-art cold-wall type single-wafer-processing apparatus for growing a gas phase silicon epitaxial wafer. FIG. 8 is a schematic diagram illustrative of a longitudinal sectional plan view of the apparatus of FIG. 7. The apparatus 1 comprises a reactor A, a gas feeder system B, heaters 6 for heating silicon epitaxial wafers, a wafer transfer system(not shown), a control system (not shown) and a vacuum system (not shown).
The reactor A is made of quartz and has the form of an elongate housing defining a reaction chamber 2a, and a disc-shaped susceptor 4 mounted within the reactor A. The susceptor 4 holds a silicon substrate wafer 3 (a silicon epitaxial wafer has the same label as the silicon substrate wafer for convenience of description hereinafter) on the top surface thereof and is rotated in a horizontal plane during epitaxy, The susceptor 4 has a shaft 4a rigidly joined thereto and extending outside the reactor A. A motor (not shown) rotates the susceptor shaft 4a.
The gas feeder system B is fluidly connected to the reaction chamber 2 of the reactor A and comprises three injectors 5 arranged transversely to the longitudinal axis of the reactor A. The three injectors 5 have a common gas feed line having mass flow controllers (MFCs) for a reactive gas.
The heaters 6 are arranged outside and along the reactor A. Each heater 6 comprises, e.g., a halogen lamp.
The reactive gas consists of the silicon source (e.g. monosilane, dichlorosilane, trichlorosilane or tetrachlorosilane), a small volume of the dopant and hydrogen for diluting the silicon source and the dopant. The injectors 5 concurrently feed flows of the reactive gas of an equal silicon source concentration and an equal dopant concentration to the main surface of the wafer 3 on the susceptor 4. The MFCs control the silicon source concentration and the dopant concentration of the reactive gas fed to the three injectors 5.
The prior-art apparatus 1 has a drawback described below. The prior-art apparatus 1 controls the temperature of the susceptor 4 so that the temperature of the periphery of the susceptor 4 and the temperature of a central part of the susceptor 4 are equal. If the temperature of the periphery of the susceptor 4 differs from the temperature of the central part of the susceptor 4, the in-plane temperature distribution of the wafer 3 is not flat nor uniform, so that an epitaxial layer experiences a thermal stress and a grown silicon epitaxial wafer 3 has slip lines.
In addition, the prior-art apparatus 1 continues to cool the wall of the reactor A during epitaxy so that the interior surface of the wall of the reactor A precludes a deposition of silicon thereto. Therefore, even if the temperature of the periphery of the susceptor 4 and the temperature of the central part of the susceptor 4 are equal, the temperature of the central part flow of the reactive gas tends to be higher than the temperature of the peripheral part flow of the reactive gas. This expands the reactive gas of the central part flow to reduce he density of the reactive gas of the central part flow and drives the reactive gas of the central part flow to the peripheral part of the reaction chamber 2a. Thus, the density of the reactive gas of the central part flow is lower than the density of the reactive gas of the peripheral flow, so that the silicon source concentration in the central flow of the reactive gas is lower than that in the peripheral flow of the reactive gas. FIG. 2 indicates that the thickness of a central part of the epitaxial layer is thinner than the thickness of a peripheral part of the epitaxial layer.
The resistivity of the epitaxial layer depends on the intensity of the influence of the automatic doping (autodoping) of the back surface of the substrate wafer. The intensity of the influence of the autodoping on the peripheral part of the epitaxial layer is higher than the intensity of that on the central part of the epitaxial layer. For example, the influence of the autodoping reduces the resistivity of an n/n.sup.+ or p/p.sup.+ epitaxial layer so that the resistivity of a central part of the epitaxial layer thereof is higher than that of a peripheral part of the epitaxial layer thereof, as shown in FIG. 3. A growth of an epitaxial layer on the substrate wafer which has an oxide layer on the overall back surface reduces the influence of the autodoping. However, even in the case, a similar tendency appears.
In order to solve the drawback described above, the temperature of the central part of the substrate wafer 3 must be higher than that of the peripheral part thereof. This causes a thermal stress in the grown epitaxial layer to produce slip lines therein.